Since accumulators of this type were first introduced, numerous constructions have been developed for shifting the position of the propelling member between accumulation and transport modes. While many of these have been very effective, they have all had one or more shortcomings which have heretofore not been overcome. One of these has been the noise incident to the frequent movement of the propelling member supporting devices, particularly when the equipment is used at higher speeds. Another problem has been that of providing a means which will shift the driven belt or propelling member without requiring either the application of a substantial force by the article contacting sensor or an intervening auxiliary power source such as a solenoid or a pneumatic actuator. Without the auxiliary power source only a limited range of article weights were capable of triggering the mechanism. The use of the auxiliary or secondary power source materially increased both complexity and cost. Since the introduction of the original accumulators, the demand for increased throughput and speed has materially increased the problem of noise and the necessity for developing drive belt or propelling member control systems which have a dampened reaction, that is, do not respond immediately each time a sensor is actuated by an article passing over it. This is particularly important in reducing the effective operating noise level of this type of accumulator as well as decreasing wear and increasing article throughput.